The Detached Dust Shell around the Massive Star HD 179821

We have used the Keck I telescope to resolve at three mid-IR wavelengths the emission from HD 179821 (= RAFGL 2343), a G-type supergiant of perhaps 30 M middle dot in circle with a detached dust shell. The shell is very approximately circular in shape with an inner diameter of approximately 3&farcs;3, corresponding to 3.0x1017 cm. We estimate that the star was losing approximately 4x10-4 M middle dot in circle yr-1 until about 1800 yr ago, when the mass loss slowed dramatically. During the past approximately 104 yr, the star has lost approximately 10% of its initial mass. The star lies about 0&farcs;35 off center and is closer to the brighter, northern hemisphere of the nebula, which can be explained if the outflow velocity Vinfinity deviates by +20% from the average in the southern hemisphere and -20% from the average in the northern hemisphere. The mass-loss rate M&d2;(straight theta) may have been inversely correlated with the outflow velocity so that the momentum outflow p&d2; was isotropic during the mass-loss phase. It also seems that M&d2;totalVinfinity was within a factor of 2 of L*/c, where L* is the current luminosity of the star; the mass loss may have been driven by radiation pressure. These results may help characterize the asymmetric circumstellar winds into which supernova explosions propagate.